We just moved into a new house and the previous owner ran two 20amp circuits with 15 amp receptacles in the garage via conduit. There are four wires, red, black, white, green shared between the two circuits. He shared the neutral between the two circuits. Is this safe? Legal? Can a ground be shared between two(or more) circuits?

I want to add another circuit to the garage as well (20amp). If multiple circuits can share a neutral, is it limited to two?

The red is stripped in a couple junction boxes so I'll be pulling new wire for that one as well. (I found these as the receptacles were not tight and I pulled them out to figure out why.)

It’s somewhat common to do that and to code. The idea is I believe the red and black are from opposite legs of the 220 and are out of phase so when one is conducting the other is down. That way the white common is only seeing the current of one at a time. It’s called a multi wire branch circuit.

Solid core in conduit doesn't sound right.
Could be a local code thing but, I can't imagine what wire was used in conduit that is solid core w/o being romex or MC.
In which case, you need to get some more feedback on the type of wire used and it's compliance in this application.

OK, I know this thread is from several months ago, and the OP hasn't been back here since June, but I'd like to clear a few things up.

-No reason to change the receptacles to 20A as suggested, unless of course they are needed for a tool or larger appliance, which is unlikely. As long as there is more than just one simplex receptacle on the 20A circuit 15A are fine. And depending on the grade/quality there is absolutely NO difference other than the prong configuration. ALL 15A duplex receptacles are rated for 20A feed-thru, even the cheap $.59 ones.

-It was also suggested that solid wire was somehow strange or not acceptable. This is very misleading. Solid wire is absolutely typical and perfectly acceptable. Solid THHN is no different than stranded in application.

I agree , we use solid # 12 THHN all the time . Easier to bend around and put under screws for receptacles or switches .

" Back - stab " wired devices were a very bad idea . I am pretty sure they are no longer made ( thankfully ) .

At one time , around here , a 200 amp meter base was the " max " . We now can use up to about a 320 amp meter base . Maybe up to close to 400 amps ?

But traditionally 200 amps is sufficient for a small to medium size home , even if all electric , except in the coldest climates . This does not include the use of instanious / tankless whole house water heaters . Which I have no experience with .